Polarized bases are needed with high intensity discharge (HID) lamps in which the arc tube is transversely mounted. HID lamps generally comprise a glass outer envelope or jacket which encloses an arc tube. The jacket has a bulbous or ellipsoidal body portion which is reduced and extended at one end into a tubular neck portion to which is attached the screw-type base for fitting the lamp in a socket.
In metal halide HID lamps, vertical operation of the arc tube is generally the preferred mode for higher efficiency and longer life. In such lamps the discharge is a constricted arc which extends along the axis of the arc tube so long as the arc tube is vertical. If the arc tube is inclined out of the vertical, internal convection currents affect the arc and displace it from the axis. In a horizontal arc tube, the arc is bowed up and may even contact the upper region of the wall, causing it to overheat while the lower extremities of the tube are underheated. The partial overheating results in poor maintenance and shorter life, while the partial underheating reduces the vapor pressure of the metal halides and causes lower efficiency and poor color rendition; tube life and efficiency can be reduced as much as 10 to 15 percent.
While vertical operation of HID lamps having the arc tube axial is most common, there are many installations wherein considerations of space or of convenience and economy require that the outer envelope of the lamp be mounted horizontally. In recent years, and primarily as a result of improvements in color rendition, metal halide lamps have increasingly been used indoors and in applications where ceiling height is limited. Fixtures for such applications mount the lamp horizontally in order to save space and this has increased the demand for metal halide lamps able to operate efficiently in this way. One suitable type of HID lamp is described in U.S. Pat. No. 4,341,975, issued July 27, 1982 of R. G. Phillipp et al and assigned to the same assignee as this invention. In that lamp the arc tube is mounted transversely to the longitudinal axis of the jacket and base. A locator such as a pin or protuberance is provided which protrudes outwardly from the thin-walled metal shell of the base and strikes a stop in the mating socket after the lamp has been screwed in almost fully home. Upon engaging the stop, the pin prevents further rotation of the lamp such that a predetermined uniform orientation, chosen to make the arc tube vertical, is achieved in all sockets. Desirably the pin engages the stop with a locking effect that tends to hold the lamp in place notwithstanding vibration.
The difficulty in providing a locating pin arises from the thinness of the metal wall, usually of brass or aluminum, and the need to have enough strength and shear resistance in the pin that it is not broken off when turned hard against the stop. Merely welding a pin on the crest of the thread does not provide enough strength. One way of achieving adequate strength is to drill a hole through the crest of the thread of the base, place a locating pin within the hole so that it extends outwardly, and then silver-solder the pin in the hole from the inside of the lamp base. Such a method is complicated and expensive from a manufacturing standpoint and also must be performed before the lamp base is attached to the outer envelope of the lamp. Another way described in copending application Ser. No. 236,595 filed Feb. 20, 1981 by H. G. Peters and assigned like this application is to weld a pin onto the crest of a thread and then form a solder fillet around the pin to provide the requisite strength and lateral support.